Nonpoint source pollution from agriculture is one of the main causes of water quality degradation. To cope with this issue, diverse policy measures have been implemented to promote farming practices that favour water quality. Among these measures, many instruments are based on the voluntary commitments of farmers. Farmers'’ participation is therefore important for water quality outcomes. To analyse this issue, we built an agent-based model of a drinking water catchment area using the social–ecological systems framework and the theory of planned behaviour. This model aims to better understand the impact of behavioural dynamics on the effectiveness and efficiency of policies targeting water pollution. Our model allows us to explore how the evolution of farming practices is influenced by, first, the behavioural characteristics of farmers and, second, the characteristics of the policy measures implemented. Our main results are threefold: 1) The characteristics of farmers influence policy effectiveness and efficiency; 2) taking into account farmers' behavioural characteristics in the definition of policies may improve the efficiency of policies; and 3) in situations where behavioural characteristics are unknown, policies combining both financial incentives and training measures are costlier but may be a better option than policies relying on a single measure.

农业面源污染是水质恶化的主要原因之一。为应对这一问题，已实施多种政策措施，以促进有利于水质的耕作方式。在这些措施中，许多工具都是基于农民的自愿承诺。因此，农民的参与对水质结果很重要。为了分析这个问题，我们使用社会生态系统框架和计划行为理论建立了一个基于代理的饮用水集水区模型。该模型旨在更好地了解行为动态对针对水污染的政策的有效性和效率的影响。我们的模型使我们能够探索农业实践的演变如何受到农民行为特征的影响，以及，二是实施的政策措施的特点。我们的主要结果有三个：1）农民的特征影响政策的有效性和效率；2）在制定政策时考虑到农民的行为特征，可以提高政策的效率；3) 在行为特征未知的情况下，结合财务激励和培训措施的政策成本更高，但可能比依赖单一措施的政策更好。